Alterations in genomic integrity often are associated with disease or with the propensity for disease. For example, many cancers are thought to arise through a series of mutations in genomic DNA, resulting in genomic instability in the form of uncontrolled cellular growth. In normal cells, damage to genomic DNA typically leads to expression of tumor suppressors, such as the cell-cycle regulator, p53. For example, damage to cellular DNA results in increased expression of p53 which arrests the cell cycle to allow repair of the damage. If the damaged DNA cannot be repaired, the cell undergoes apoptosis, thus preventing the accumulation of additional mutations in daughter cells. If however, there is a mutation in the p53 gene itself (or in another cell cycle regulator), damaged cells will proceed through the cell cycle, giving rise to progeny in which additional DNA mutations will go unchecked. It is the accumulation of these mutations that is the hallmark of many cancers.
The process of apoptosis is important not only in the regulation of cellular metabolism, but also in inhibiting oncogenesis. As cells undergo apoptosis, the nucleus becomes small and fragmented. Nuclear DNA is digested into spindle fragments that are generally no larger than about 200 base pairs. As the process continues, usually through multiple pathways, the cell membrane breaks down, and cellular contents are metabolized. As a result, cells that have the potential to enter the multi-step pathway leading to cancer are eliminated.
Many cancers are curable if detected early in their development. For example, colorectal cancers typically originate in the colonic epithelium, and are not extensively vascularized (and therefore not invasive) during early stages of development. The transition to a highly-vascularized, invasive and ultimately metastatic cancer commonly takes ten years or longer. If the presence of cancer is detected prior to extensive vascularization, surgical removal typically is an effective cure. However, colorectal cancer is often detected only upon manifestation of clinical symptoms, such as pain and black tarry stool. Generally, such symptoms are present only when the disease is well established, and often after metastasis has occurred. Early detection of colorectal cancer therefore is important in order to significantly reduce its morbidity.
Invasive diagnostic methods, such as endoscopic examination, allow direct visual identification, removal, and biopsy of potentially-cancerous tissue. Endoscopy is expensive, uncomfortable, inherently risky, and not a practical tool for early diagnosis.
Established non-invasive screening methods involve assaying stool samples for the presence of fecal occult blood or for elevated levels of carcinoembryonic antigen, both of which are suggestive of the presence of colorectal cancer. Additionally, recent developments in molecular biology provide methods of great potential for detecting the presence of a range of DNA mutations indicative of colorectal cancer. The presence of such mutations can be detected in DNA found in stool samples during various stages of colorectal cancer. However, stool comprises cells and cellular debris from the patient, from microorganisms, and from food, resulting in a heterogeneous population of cells. This makes detection of small, specific subpopulations difficult to detect reliably.
There is a need in the art for additional non-invasive methods for early diagnosis of cancer that will detect characteristics indicative of the presence of cancer.